1. Field of the Invention
The present invention relates to the field of semiconductor devices, and in particular, to a method of metal gate work function engineering for surface p-channel devices that eliminates the need for nitrided gate oxides.
2. Description of the Related Art
As is known in the art, MOSFET devices, such as CMOS, have a source and drain region separated by a gate region, all typically formed on a surface of a semiconductive, silicon substrate. Individual devices are typically isolated from each other with isolation structures. Overlying the gate region is a gate stack.
The gate stack enables an overlying conductor to apply a biasing voltage through the gate stack to selectively create and disable a conductive channel between the source and drain regions. Gate stacks typically are multi-layered structures designed to electrically isolate the conductor from the substrate such that minimal current flows through the gate terminal. A dielectric layer, such as silicon oxide, is typically placed above the semiconductor substrate. Then, an interconnecting conductive layer is placed above the dielectric for contact to the conductive layer. The interconnecting conductive layer provides a low resistance, preferably ohmic, contact with the conductor. The composition of the interconnecting layer is typically selected to exhibit good adhesion with both the conductive layer and the underlying dielectric layer and to be compatible with other processing steps and materials. The layers are also chosen to mitigate undesired reactions with adjacent layers in subsequent heat steps that take place during the device fabrication and packaging as well as during use.
The amount of biasing voltage required to enable/disable the conductive channel under the gate is referred to as the threshold voltage (V). An additional design goal for most applications is that the threshold voltages for the NMOS and PMOS devices be complementary. Achieving complementary threshold voltages typically requires that the work function of the gate stack overlying NMOS devices be lower than the underlying p-type doped semiconductor and higher than the underlying n-type doped semiconductor in PMOS devices.
Achieving differential work functions in the gate stacks overlying the NMOS and PMOS regions places the respective Fermi levels closer to mid-gap between the gate stack and the underlying doped substrate. The known alternative to forming differential work functions in the gate stacks is to modify the work functions of the underlying channels regions through doping. Effectively doping the channel regions typically requires relative high doping levels which increases the bulk conductivity and leads to higher undesirable leakage currents.
One known prior art method of forming a gate stack, illustrated in FIG. 1 for a CMOS device, consists of layering a refractory metal conductor over an interconnecting refractory metal nitride over a dielectric layer on the surface of a semiconductive, silicon substrate. In this example, the materials are W/TiNx (x less than 1)/SiOH/Si or W/WNx (x less than 1)/SiOH/Si. The tungsten (W) metal provides low bulk resistivity while tolerating subsequent high temperature processing steps. The SiOH is a nitrided gate oxide. The gate oxide is nitrided to inhibit the adjacent metal rich TiNx or WNx from reacting with underlying silicon dioxide without nitride in subsequent heat steps. The heat steps can induce metal atoms to migrate and react with the gate oxide where the metal atoms oxidize thus consuming oxygen atoms from the oxide. This results in the gate oxide SiO2 being transformed into SiOx (x less than 2) which is a less effective insulator and can result in dielectric breakdown which would result in device failure. Whereas the nitrided gate oxide resists this oxygen consumption and retains good insulating characteristics.
A drawback, however, is that the nitrided gate oxide, SiOH, layer in the gate stack is known to degrade the transconductance (gm) characteristics of the devices formed with this method. Transconductance is a measure of the small signal amplification of a MOSFET device. Transconductance is the partial differential of drain current with respect to gate voltage and is dependent on the gate capacitance. The transconductance is a measure of the speed of the device and, as is well understood in the art, faster device speed is a desirable feature. The gate stack is preferably formed with as low a junction capacitance as possible in order to increase switching speed. The partially nitrided silicon oxide has a higher dielectric constant that pure silicon dioxide and thus gives an increased capacitance, which correspondingly lowers the device speed.
This prior art process then involves applying photoresist to selectively mask the PMOS regions. Nitrogen is then implanted into the TiNx or WNx (x less than 1) areas of the NMOS regions. The work function of the gate stack over the NMOS regions is reduced with increasing nitrogen content in the titanium or tungsten nitride layers. The TiNx has a relatively large work function and TiN is near mid-gap. However, this type of metal nitride based metal gate structure has some disadvantages. In particular, the metal rich TiNx (x less than 1) can react with an adjacent tetraethylorthosilicate (TEOS) spacer during subsequent TEOS deposition.
In a similar manner to that previously described with respect to the nitrided gate oxide, titanium atoms can react with the SiO2 from the TEOS. This reaction consumes oxygen atoms from the silicon oxide resulting in a silicon rich, nonstoichiometric SiOx (x less than 2). The SiOx is a less effective insulator than the silicon oxide and can result in incomplete isolation between individual devices and circuit failure. For this reason, typically only a Nit spacer can be used as a Nit spacer is resistant to reaction with the titanium.
However, a Nit spacer is undesirable from the perspective of the increased dielectric constant of the Nit compared to the silicon dioxide of the TEOS spacer. The approximately doubled dielectric constant of the Nit spacer results in fringing fields that are approximately twice as intense as the fringing fields around a comparable TEOS spacer under a comparable electric field. The higher fringing fields around Nit spacers can cause dielectric breakdown of the gate oxide particularly near the edges of the gate oxide. As is well understood in the art, breaking down the gate oxide damages the device and typically causes failure of the device. Thus, a TEOS spacer is more robust than a comparable Nit spacer.
A further drawback to the previously mentioned gate materials arises from the internal stress of the materials and their adhesive characteristics. In particular, any deposited material will possess some degree of residual stress. The stress may be either tensile or compressive. Tensile stress is relieved by the material contracting while compressive stress is relieved by the material expanding. If the stress in a deposited layer exceeds its adhesion to adjacent layers, the layer may physically peal off the underlying/overlying surfaces. This would have dramatic negative effects on the reliability of metal lines and other device structures. Large stress may also give rise to void formation during subsequent thermal cycling. The stress provides an impetus for grain boundary diffusion. Suitable materials must possess sufficient adhesion to adjacent layers to inhibit separation due to stress.
The TiSixNy previously described has good adhesion characteristics, however it has a very high stress level ( greater than 15GD/cm2). The WSixNy has low stress ( less than 5GD/cm2), however, it suffers from poor adhesive characteristics. As previously mentioned, poor adhesion can also result in a layer becoming dislodged from adjacent layers and resultant device failure.
From the foregoing, it can be appreciated that there is a need for a method of forming a gate stack of materials with good adhesion characteristics and low stress. There is also a need for a method of forming a gate stack with improved transconductance performance and complementary threshold voltage characteristics.
The aforementioned needs are satisfied by the present invention which, in one aspect, is a semiconductor device comprising a semiconductive substrate, a dielectric layer positioned on the semiconductive substrate, and an interconnecting layer positioned on the dielectric layer wherein the interconnecting layer comprises a metal silicide wherein the metal is matched with the silicide so that the metal is inhibited from reacting with the dielectric layer and also wherein the metal is selected such that selective nitrification of the metal silicide lowers the work function of the metal silicide. In one embodiment, the metal is further selected such that the metal silicide and nitrided metal silicide exhibit improved adhesion to the adjacent layers.
In another aspect, the invention further comprises a conductive layer positioned on the interconnecting layer such that the dielectric layer, the interconnecting layer, and the conductive layer together define a gate stack. In one embodiment, the metal comprises tantalum, the semiconductive substrate comprises silicon, and the dielectric layer comprises silicon dioxide. In other embodiments, the conductive layer comprises tungsten, cobalt silicide, or nickel silicide.
An additional aspect of the invention is a method of selectively setting the work functions of semiconductor materials comprising forming a dielectric layer on a semiconductive substrate, forming an interconnecting layer atop the dielectric layer wherein the interconnecting layer comprises a metal silicide wherein the metal is matched with the silicide in a stable phase such that the metal is inhibited from reacting with the dielectric layer, selectively masking portions of the interconnecting layer, and reacting unmasked regions of the interconnecting layer with nitrogen so as to transform the unasked regions of the interconnecting layer to reacted metal-silicon nitride such that the masked regions of the interconnecting layer remain unreacted metal silicide wherein the metal is selected such that the metal-silicon nitride has a lower work function than the unreacted metal silicide. In one embodiment, the metal is further selected so that the metal silicide exhibits improved adhesion to adjacent layers.
Another embodiment further comprises forming a conductive layer atop the interconnecting layer wherein the dielectric layer, the reacted and unreacted interconnecting layers, and the conductive layer together define a gate stack of a semiconductor device. In one embodiment of the method, the metal used is tantalum, the semiconductive substrate comprises silicon, and the dielectric layer comprises silicon dioxide. In particular embodiments, the conductive layer comprises tungsten, cobalt silicide, or nickel silicide.
Yet another aspect of the invention is a method of establishing differential work functions in gate stacks overlying complementarily doped regions of semiconductor devices comprising forming a dielectric layer on a semiconductive substrate having complementary regions doped n- and p-type, forming an interconnecting layer atop the dielectric layer wherein the interconnecting layer comprises a metal silicide wherein the metal is matched with the silicide in a stable phase such that the metal is inhibited from reacting with the dielectric layer, selectively masking portions of the interconnecting layer, reacting unmasked regions of the interconnecting layer so as to lower the work function of the unmasked regions such that the masked regions of the interconnecting layer remain unreacted, and selectively removing the masking and portions of the interconnecting layer and the dielectric layer so as to form the gate stacks. In one embodiment, the masking is applied to the n-type regions and reacting comprises nitrification. In certain embodiments, the metal is a middle transition metal and, in one particular embodiment, the metal is tantalum.
The improved gate engineering of the present invention provides selective nitrification of regions of a semiconductor device to selectively modify the work functions of the selected regions to provide complementary work functions in the gate stacks. The metal silicides and metal silicon nitrides of one embodiment of the invention exhibit low reactivity and good adhesion with adjacent materials as well as low internal stress. The improved gate engineering also enables a non-nitrided gate oxide which is unreactive with adjacent layers throughout subsequent heat cycling. The non-nitrided gate oxide also exhibits lower junction capacitance and increased device speed as compared to devices which require a nitrided gate oxide.
These and other objects and advantages of the present invention will become more fully apparent from the following description taken in conjunction with the accompanying drawings.